PROJECT SUMMARY Rates of anxiety and depression in youth are substantial, causing a major unmet need for effective interventions. Although some progress has been made in preventing these internalizing problems in adolescents, further research is needed that specifically targets theoretically and empirically supported risk processes. An important and salient risk factor found to increase the likelihood of anxiety and depression is negative affectivity ? a partially heritable trait propensity to experience and express more frequent, intense, and enduring aversive emotional states. The proposed randomized controlled prevention trial builds on our finding from our longitudinal study that elevated levels of negative affectivity during adolescence prospectively predicted internalizing disorders in early adulthood (Zinbarg et al., 2016); moreover, this relation was mediated by changes in momentary negative affect (mNA) measured with ecological momentary assessment (EMA) (Adam et al., 2018). The first phase (R61) of the proposed selective prevention trial will test whether an app-based, coach-supported mindfulness intervention as compared to an assessment-only control reduces momentary negative affect, measured with ecological momentary assessment (EMA), in 120 adolescents (age 12-16) at- risk based on their having high levels of trait negative affectivity. EMA will be used to measure average daily mood, (the ?Target?) collected six times a day across three days at pre-, mid-, and post- intervention. ?Target? engagement will be defined as a medium effect size (>.40) in the comparison of youth randomized to MBI versus control on the target ? momentary negative affect ? at post-test, adjusting for pre-test levels. We also will assess the dose-response relation by testing the association between number of sessions and exercises completed with changes in momentary negative affect and weekly mood ratings. In the second phase (R33), we will conduct a replication trial with a new sample of 360 at-risk (i.e., high trait negative affectivity) youths (ages 12-16) randomized to one of three conditions ? MBI, a nonspecific control, or an assessment-only control. Youth will be evaluated with regard to the target (i.e., mNA), internalizing symptoms and disorders, and functioning (e.g., social, academic) at baseline and post-intervention (R61 and R33), and at a 6-month follow-up (R33). Finally, in the R33 we will test if significant reductions in momentary negative affect are associated with improvements (or less worsening) in internalizing symptoms and fewer onsets of internalizing disorders.